In order to achieve the increasingly higher recording densities required by today's magnetic recording media, recording wavelengths are being shortened. However, unless the particle size of the magnetic powder is much smaller than the length of the region for recording the short-wavelength signal, a distinct magnetic transition cannot be produced, which, in practical terms, makes recording impossible. Thus, the particles of the magnetic powder have to be sufficiently smaller than the recording wavelength.
Achieving higher recording density also requires increasing the resolving power of the recording signal, so it is also important to reduce magnetic recording medium noise. Particle size is a major factor in noise and the smaller the particle size is made, the more advantageous from the viewpoint of noise reduction. Accordingly, magnetic powder for high-density recording applications has to have a sufficiently small particle size to achieve the required noise reduction.
However, as the particles get smaller, it becomes more and more difficult for the particles to continue to exist as independent particles. For example, in the case of the metal magnetic powder generally used in a data storage tape, extreme refinement of the particle size makes the powder susceptible to sintering during the reduction stage of the production process. Sintering increases the average particle volume, which is undesirable because the larger sintered particles become a source of noise, and also degrades the magnetic properties of the powder by deforming the particle shape. In addition, when the powder is used to produce magnetic tape, the enlarged particles degrade dispersibility and cause loss of surface smoothness. The magnetic powder therefore becomes unsuitable for use in high recording density media. While a magnetic powder needs to have good magnetic properties to be suitable for a high-density recording medium, it also has to exhibit good powder properties during the tape manufacturing process, such as dispersibility, average particle volume, particle size distribution, specific surface area, tap density and so forth.
Known magnetic powders with excellent magnetic properties suitable for high density recording media include, for example, that taught by JP 2000-6147A (Ref. 1), which is a ferromagnetic metal powder whose properties include: major axis length of 30-120 nm, axial ratio of 3-8, Hc of 79.6˜318.5 kA/m (1,000-4,000 Oe), and σ of 100˜180 Am2/kg (100-180 emu/g).
Further, JP 10-69629A (Ref. 2) teaches a magnetic powder for achieving superior magnetic properties of a high quality that is composed of Fe containing 5-50 at. % of Co, 0.1-30 at. % of Al, 0.1-10 at. % of rare earth elements (defined to include Y), not more than 0.05 wt % of Periodic Table group 1 a elements and not more than 0.1 wt % of Periodic Table group 2 a elements and has Hc of 95.5˜238.8 kA/m (1,200-3,000 Oe) and as of 100˜200 Am2/kg (100-200 emu/g).
JP 2003-263719A (Ref. 3) teaches a magnetic powder compatible with MR heads that is composed of acicular particles comprised primarily of Fe including Co, Al, R (rare earth elements; including Y) and oxygen within prescribed ranges, wherein Co/Fe=10-50 at. %, solid state Al/(Fe+Co)=5-50 at. %, R/(Fe+Co)=2-25 at. %, oxygen≦25 wt. %, average major axis length of the acicular particles=25-80 nm, and saturation magnetization σs=10˜130 Am2/kg (10-130 emu/g).
As an iron nitride system magnetic powder suitable for high density recording media, WO 03/079333A1 (pamphlet; Ref 4) teaches a rare earth element-iron nitride system magnetic powder composed of substantially spherical or ellipsoid particles and states that, despite being composed of fine particles of around 20 nm (average particle volume of 4,187 nm3), the rare earth element-iron nitride system magnetic powder having Fe16N2 as its main phase has a high coercive force of 200 kA/m (2,512 Oe) or greater and high saturation magnetization owing to its small BET specific surface area, so that the recording density of a coated-type magnetic recording medium can be dramatically enhanced by using the rare earth element-iron nitride system magnetic powder.
The need for tape media of higher recording density continues to increase, however, and this in turn has created a need for the development of magnetic powders capable of responding to this need. A high C/N is indispensable to the realization of high recording density, i.e., a tape is required that is low in noise (N) and high in output (C). A magnetic powder small in particle volume and excellent in magnetic properties is preferable for producing such a medium. In recent years, advances in magnetic head technology have led to the development of GMR and other high-sensitivity heads capable of reading data recorded at low magnetization. Although this makes output less of a concern when using a magnetic powder of low magnetization σs, it aggravates the problem of noise, because when a high-sensitivity head is used even slight noise is detected as large noise to markedly degrade the C/N ratio. High recording density media must therefore be designed giving attention to both medium and head, with more focus on lowering noise than enhancing output.
However, efforts made to reduce particle size at the starting powder stage are frequently outweighed by the effects of sintering occurring at the reduction stage of the magnetic powder production process. The particle size of metal magnetic powders currently used in practical applications is around 45-60 nm (average particle volume: 5,000-8,000 nm3). In contrast, the average particle volume required by a low-noise medium is 4,000 nm3 or less, preferably 3,000 nm3 or less, but no practical magnetic powder of such adequately small average particle volume has yet been developed. When particles sinter together at the reduction stage, the presence of large particles locally within the magnetic powder increases particle-induced noise and also adversely affects roughness during tape manufacture. This makes production of low-noise tape impossible.
Sintering is prevented chiefly by 1) changing the composition of the starting powder (increasing the amount of sintering inhibitor used) and 2) lowering the reduction temperature to which the metallic iron is exposed. The first method of increasing the amount of nonmagnetic sintering inhibitor is undesirable because it increases noise by reducing the number of magnetic particles per unit volume. The second method is undesirable because lowering the reduction temperature not only has the desired effect of reducing sintering but also simultaneously lowers the particle reduction rate, which leads to problems such as that the proportion of the grain boundary rises because crystal grain growth within the particles is inhibited and the magnetic properties are markedly degraded by the occurrence of magnetic poles and the like owing to increased irregularity of the particle surfaces. These drawbacks of the conventional methods point up the need for development of a new sintering inhibiting technique that enables particle refinement without degrading magnetic properties.